Very low frequency location finding system

ABSTRACT

To display at a command post the positions of all vehicles in a tactical area, each vehicle receives two very low frequencies (VLF&#39;&#39;s) of an &#39;&#39;&#39;&#39;Omega&#39;&#39;&#39;&#39; system, and retransmits the phase information thereof by a high frequency link to the command post where a general purpose computer determines the direction and the distance to each remote vehicle.

Unite Seams asem Bickel et a1.

[451 July 25,1972

[54] VERY LOW FREQUENCY LOCATION FINDING SYSTEM [72] Inventors: John E.Blckel; Clemens l-l. Freres, both of San Diego; Earl E. Gossard, Delmar;James E. Brltt, San Diego; Norman R. Ortweln, San Diego; Eric R.Swanson, San Diego, all of Calif.

[73] Assignee: The United States of America as represented by theSecretary of the Navy [22] Filed: April 29, 1969 [21] Appl.No.: 823,239

[52] U.S. Cl. ..343/105 R, 343/112 R, 343/112 TC [51] Int. Cl ..G0ls1/30 [5 8] Field of Search [56] References Cited UNITED STATES PATENTS3,160,846 12/1964 Gustafson et a1. ..343/1 12 X 3,430,234 2/1969 Wright..343/l05 X 3,471,856 10/1969 Laughlin, Jr. et a1. ..343/105 X PrimaryExaminer-Rodney D. Bennett, Jr.

Assistant Examiner-Richard E. Berger AttorneyG. J. Rubens, R. S.Sciascia, J. W. McLaren and T. M. Phillips [5 7] ABSTRACT To display ata command post the positions of all vehicles in a tactical area, eachvehicle receives two very low frequencies (VLF s) of an Omega" system,and retransmits the phase information thereof by a high frequency linkto the command post where a general purpose computer determines thedirection and the distance to each remote vehicle.

4 Claims, 4 Drawing figures I DIFF. DISTANCE i (0.51111. ACCURACYFTIIITIEJUW I972 SHEET 1 [IF 2 DIFF. DISTANCE (0.5 mi. ACCURACYELLIPTICAL LINE-OPPOSITION M U 0 F R O F V L F MODULATION H. F. RELAYS(S TOM) ELLIPTICAL LINE-OF-POSITION INVENTORS V TORE' rs FOR FOCI -T MPATEEDIIIIzs I972 3.6801 18 SHEET 2 UF 2 w ,y T T 22 v FILTER 20 26 2a30 32 f l FILTER 2 M00 m V V V HF k f, TRACKING TIS+SM 50 DEMOD FILTER mCOUNTER GATE f TRACKING M START FILTER 46 40 STABLE f TRACKING Ta5+$M 5208C FILTER m COUNTER GATE f TRACKING z START FILTER 56 R- 5 a 6 7 3 5 74 3 I J COMPUTER G -o INVENTORS JOHN E. BIC EL CLEMENS H. FRERES EARL E.GOSSARD BY JAME BRITT S E NORMAN R. ORTWEIN ERIC RI SWANSON VERY LOWFREQUENCY LOCATION FINDING SYSTEM STATEMENT OF GOVERNMENT INTEREST Theinvention described herein may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION Generally, radio navigating systems are oflittle value in controlling the military traffic in a tactical area anddo not respond well to the central command. Because of the speed of jetfighters they are over the radio horizon much of the time and out ofcontact with the central command station. Homing systems can guidefighters to their carriers, but little more. All navigating equipmentwhich must be airborne is prohibitively expensive in terms of reducedpayloads. This fact is particularly true of the long wave radio gearused in the Omega" system where to 14 kilohertz are used. At presentthere are several of a total of eight such transmitters operative aboutthe earth. They lay down a grid of guidelines. Dipoles for such lowfrequencies are 7 miles long.

The object of this invention is to minimize the weight of radio gear tobe carried in the airplanes in a battle area and at the same timeprovide the area commander with a display of the position ofall friendlyunits in his command SUMMARY OF THE INVENTION The objects of thisinvention are attained by a repeater for receiving the VLF positioninformation signals, modulating the information on a high frequencycarrier, and retransmitting the modulated carrier to the master orheadquarters station where the phase information is detected anddisplayed. This provides the headquarters command with a picture of allunits in the area and at the same time relieves the airborne unit of thebulkiness and weight of phase detectors and computing equipment forposition determination.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a segment ofthe earths surface and the deployment of equipment of the system of thisinvention.

FIG. 2 is a diagram showing the geometric problem solved by the systemofthis invention.

FIG. 3 is a block diagram of the airborne equipment of this invention.

FIG. 4 is a block diagram of the master station equipment of thisinvention.

PREFERRED EMBODIMENTS OF THE INVENTION A substantial portion of theearths surface is presently covered by a grid of imaginary lines laiddown by OMEGA VLF transmitters located at various fixed points about theglobe. Each transmitter is connected to a long antenna and broadcastssubstantially omnidirectionally. The low frequencies follow thecurvature of the earth and are not substantially attenuated beyond thehorizon as in the case of higher frequencies. In FIG. 1 is shown two ofthe fixed low frequency transmitters labeled T, and T The tacticalsituation assumed for describing this invention will be that where theairplane carrier 10 is at some great distance from the two transmittersand comprises the command post and homing station for a number ofairplanes 12. According to this invention, a large portion of theelectronic gear necessary in an operating squadron is transferred to thecarrier 10.

Since the error in any radio navigating system is generally a fixedpercent of the range, it is desirable to eliminate the error caused bygreat distances between the carrier 10 and the radio transmitters T, ndT In tactical situations, it is more impor tant to accurately know therange and direction of each of the airplanes 12 with respect to itslanding strip on the carrier. While the triangular, geometric problemssolved by the system of this invention include thegreat rangesmentioned, these ranges so appear in the computation that they generallycancel out. The output ofa carrier based computer will include onlyerror attributable to the short ranges between the carrier and itsairplanes.

According to an important feature of this invention, the geometry of thetactical problem of FIG. I can be laid out as illustrated in FIG. 2where the two land-based OMEGA transmitters T, and T and the masterstation, M, preferably aboard the carrier 10, and the slave station, S,preferably airborne, define two triangles. One side, S-M, of each of thetriangles is common to the other. Only two OMEGA very low frequency(VLF) transmitters are required, as distinguished from the usual threetransmitters required in Omega" type navigation. Further, allcomputation and phase tracking is done at the headquarters unit M. Theposition of the slave S with respect to M is the only. unknown. Thelines-of-position of S with respect to M are elliptical, asdistinguished from the more common hyperbolic systems as OMEGA or LORAN.The position of the slave is at the intersection of two ellipses asshown in FIG. 2. An ellipse is defined as the locus of a point for whichthe sumof its distances from two fixed points is constant. Thus, for Son any ellipse drawn about the foci T, and M the sum (T,S+SM) isconstant, and for foci T and M, the sum (T S+SM) is constant. Theinvention described herein measures the difference [(T,S+SM)*T,M] and[(T S+SM)-T M] Since by independent means such as OMEGA, T,M and T M areknown, (T,S+SM) and (T S+SM) may be determined. The angle B shown inFIG. 2 is also known. Therefore, with the law of cosines the trianglesT,MS and T- MS may be determined giving the values for MS, 9, and 0noting from the figure that 0, [3+0 The equations found by the law ofcosines may be written.

(T28 SMF mm A telemetry path is established between the slave S and themaster M which path might be a high frequency ground wave of a frequencyof perhaps 2 or more megacycles per second modulated in amplitude,frequency or in phase by the very low frequency (VLF) signals receivedat the slave station from the transmitters T, and T For the narrowbandwidth permitted by the system, practical ranges out to 300 miles isrealized. The higher the telemetry frequency, the lighter and smallercan be the equipment necessary to repeat and pass on the phaseinformation received at the airplane slave station S.

More specifically, the block diagrams of FIGS. 3 and 4 illustrate oneembodiment of this system. On the airplane is carried the repeaterincluding the antenna 20 coupled to filters 22 and 24 for selecting thesignals f, and f from the fixed, remote transmitters T, and T Thesesignals are suitably amplified, limited, applied to the linear summingdevice 26, and modulated at 28 on the high frequency carrier, F,generated in transmitter 30. The omnidirectional antenna, such as a whip32, now radiates the F if, and F if, This radiated signal is received atthe master station on antenna 34 and is demodulated in the receivingcircuits 36 to recover thef, andf signals which are in turn separated bythe tracking filters 38 and 40.

Additionally, the same signalsf andf radiated by the fixed transmittersT, and T are received directly on antenna 44 and are separated intracking filters of 46 and 48. Now the magnitude of the differencebetween the direct path, TM, and the indirect path TS SM, appears at theoutputs of the tracking filters and conveniently these magnitudes can bemeasured by the gates 50 and 52 which will permit the output of anystable high frequency clock source 54 to flow into the countingregisters 56 and 58 to display the numerical magnitude of the differencein the sides of the triangles of FIG. 2.

Since the length of the line T,M, FIG. 2, is always equal to or lessthan the sum (T,S+SM) of the lines T,S and SM, the

signal f, received over the direct path will always arrive at the masterstation ahead of the signal arriving over the indirect path; hence theT,M signal is applied to the start circuit of the counter gate 50.Likewise, the directly received signalf is applied to the start circuitof counter 52. Each start circuit opens the gate and permits the flow ofhigh frequency pulses from the source 54 into the registers 56 and 58.Then the later in directly received signals are applied to the stopcircuits of the counters and the differences now of the direct andindirect paths and shown on the counters. These two magnitudes canpreferably be applied to the computer 60 which will isolate the desiredunknown SM.

Many specific computer programs may be adopted for solving the geometricproblem of FIG. 2 where it is desired to know the direction as well asthe range from M to each of a large number of airplanes S. This requiresinstant computation and display.

For any value of time difference in the direct and indirect paths fromT, to M, there is a large number of values ofTS and MS. The timedifference, or distance difference, measured by the counter gate 50 andits display 56 expresses not a position but a line of position. Toobtain a fix along any line, a second line of position intersecting thefirst line must be established. Establishing the fix position of point Scan be accomplished in different ways. Sets of graphs of elliptic curveshaving series of progressive ratios of major-minor axes corresponding tothe time-difierence numbers displayed at 56 and 58 may be used.

Alternatively, the two lines of position can be computed directly in thecomputer 60 of any general purpose capability, merely by feeding thedistance information T 8 SM, T,S SM, T M, T,M, properly digitalized intothe computer.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

l. A system for locating point S with respect to point M comprising:

two remote broadcasting transmitters T, and T radiating frequenciesf,and f respectively, said transmitters being at known fixed spacedlocations for laying down in the area of points S and M a grid ofintersecting lines of positron,

a radio repeater at said point S for receiving said frequenciesf, and fand rebroadcasting said frequencies on a carrier wave of uniquefrequency, F,,

means at said point M for receiving all frequenciesf,,f and F,, and

means at said point M for measuring the time phase of signalsf, and freceived directly from said two remote transmitters T, and T means atsaid point M for measuring the time phase of signal f, andf receivedindirectly from said two remote transmitters T, and T through saidrepeater, and

means at said point M for computing from said time phases the distancesfrom M to T,, to T and to S.

2. A system for controlling a squadron of airplanes from a relativelystationary command post, said system comprising;

two fixed omnidirectional broadcasting stations, T, and T radiatingfrequencies f, and f respectively, said broadcasting station being atknown fixed spaced locations for laying down in the area of theairplanes and the command post a grid of intersecting lines of position,

a slave radio relay station, S, carried on each of said airplanes, eachslave station including a HF transmitter and a radio receiver coupleddirectly to and modulating said HF transmitter for receiving saidfrequencies f, and f and rebroadcasting said frequencies on a carrierwave of unique frequency, F,,

a command post comprising a master radio receiver, M for receiving allfrequenciesf,,f and F,, a computer coupled to the output of said masterreceiver,

said computer including means for measuring from said master station, M,the time phase of signals f, and f received directly from saidbroadcasting stations, the time phase of signals f, and f receivedindirectly from said relay stations and for computing from said timephases the distances from said command post M, to the slave stations, S.

3. In the system defined in claim 2, the radio repeater of said slavestation S comprising;

a receiving antenna and selective filters for isolating said frequenciesf, and f said HF transmitter generating the frequency F which isrelatively high compared with saidf, orf and a modulator coupled betweensaid filters and said transmitter for modulating said frequency F withf, and f 4. In the system defined in claim 2, said master stationreceiver comprising;

an antenna system and demodulator for deriving frequenciesf, andfreceived on said carrier F,

filter means coupled to said demodulator for separating f,

from f antenna means and filter for receiving and separatingf, and

f received directly from said transmitters T, and T

1. A system for locating point S with respect to point M comprising: tworemote broadcasting transmitters T1 and T2 radiating frequencies f1 andf2, respectively, said transmitters being at known fixed spacedlocations for laying down in the area of points S and M a grid ofintersecting lines of position, a radio repeater at said point S tforreceiving said frequencies f1 and f2, and rebroadcasting saidfrequencies on a carrier wave of unique frequency, F1, means at saidpoint M for receiving all frequencies f1, f2, and F1, and means at saidpoint M for measuring the time phase of signals f1 and f2 receiveddirectly from said two remote transmitters T1 and T2, means at saidpoint M for measuring the time phase of signal f1 and f2 receivedindirectly from said two remote transmitters T1 and T2 through saidrepeater, and means at said point M for computing from said time phasesthe distances from M to T1, to T2, and to S.
 2. A system for controllinga squadron of airplanes from a relatively stationary command post, saidsystem comprising; two fixed omnidirectional broadcasting stations, T1and T2, radiating frequencies f1 and f2, respectively, said broadcastingstation being at known fixed spaced locations for laying down in thearea of the airplanes and the command post a grid of intersecting linesof position, a slave radio relay station, S, carried on each of saidairplanes, each slave station including a HF transmitter and a radioreceiver coupled directly to and modulating said HF transmitter forreceiving said frequencies f1 and f2, and rebroadcasting saidfrequencies on a carrier wave of unique frequency, F1, a command postcomprising a master radio receiver, M for receiving all frequencies f1,f2, and F1, a computer coupled to the output of said master receiver,said computer including means for measuring from said master station, M,the time phase of signals f1 and f2 received directly from saidbroadcasting stations, the time phase of signals f1 and f2 receivedindirectly from said relay stations and for computing from said timephases the distances from said command post M, to the slave stations, S.3. In the system defined in claim 2, the radio repeater of said slavestation S comprising; a receiving antenna and selective filters forisolating said frequencies f1 and f2, said HF transmitter generating thefrequency F which is relatively high compared with said f1 or f2, and amodulator coupled between said filters and said transmitter formodulating said frequency F with f1 and f2.
 4. In the system defined inclaim 2, said master station receiver comprising; an antenna system anddemodulator for deriving frequencies f1 and f2 received on said carrierF, filter means coupled to said demodulator for separating f1 from f2,antenna means and filter for receiving and separating f1 and f2 receiveddirectly from said transmitters T1 and T2.